Facebook is an online social networking service. Its name comes from a colloquialism for the directory given to students at some American universities.^[7] Facebook was founded on 4th February 2004 by Mark Zuckerberg with his college roommates and fellow Harvard University students Eduardo Saverin, Andrew McCollum, Dustin Moskovitz and Chris Hughes.^[8] The founders had initially limited the website's membership to Harvard students, but later expanded it to colleges in the Boston area, the Ivy League, and Stanford University. It gradually added support for students at various other universities before it opened to high-school students, and eventually to anyone aged 13 and over. Facebook now allows anyone who claims to be at least 13 years old to become a registered user of the website.^[9]

Users must register before using the site, after which they may create a personal profile, add other users as friends, exchange messages, and receive automatic notifications when they update their profile. Additionally, users may join common-interest user groups, organized by workplace, school or college, or other characteristics, and categorize their friends into lists such as "People From Work" or "Close Friends". As of September 2012, Facebook has over one billion active users,^[10] of which 9% are fake.^[11] Facebook (as of 2012) has about 180 petabytes of data per year and grows by over half a petabyte every 24 hours.^[12]

In May 2005, Accel partners invested $12.7 million in Facebook, and Jim Breyer^[13] added $1 million of his own money. A January 2009 Compete.com study ranked Facebook the most used social networking service by worldwide monthly active users.^[14] Entertainment Weekly included the site on its end-of-the-decade "best-of" list, saying, "How on earth did we stalk our exes, remember our co-workers' birthdays, bug our friends, and play a rousing game of Scrabulous before Facebook?"^[15] Facebook eventually filed for an initial public offering on February 1, 2012; it is headquartered in Menlo Park, California.^[16] Facebook Inc. began selling stock to the public and trading on the NASDAQ on May 18, 2012.^[17] Based on its 2012 income of US$5 billion, Facebook joined the Fortune 500 list for the first time on the list published in May 2013, being placed at position 462.^[18] Facebook is considered the 5th most successful startup company of all time, by market capitalization, revenue, and growth.^[19]

In 2012, Facebook was valued at $104 billion, and by January of 2014 its market capitalization had risen to over $134 billion.^[20][21] At the end of January 2014, 1.23 billion users were active on the website every month, while on December 31, 2013, 945 million of this total were identified by the company as mobile users. The company celebrates its tenth anniversary in the week beginning February 3, 2014.^[22]In a January 2014, during the week previous to the company's tenth anniversary, chief operating officer of Facebook, Sheryl Sandberg, clarified: "He [Mark] always said Facebook was started not just to be a company, but to fulfill a vision of connecting the world.

Users can create profiles with photos, lists of personal interests, contact information, and other personal information. Users can communicate with friends and other users through private or public messages and a chat feature. They can also create and join interest groups and "like pages" (called "fan pages" until April 19, 2010), some of which are maintained by organizations as a means of advertising.^[141] Facebook has been prompted to add a "third gender", "other", or "intersex" tab in the gender option which contains only male and female.^[142] Facebook refused and said that individuals can "opt out of showing their sex on their profile".^[143] A 2012 Pew Internet and American Life study identified that between 20–30% of Facebook users are "power users" who frequently link, poke, post and tag themselves and others.^[144] The user page is set up in a minimal fashion with blue as the main color. This was done because Zuckerberg is red-green colorblind.^[145]

On June 13, 2009, Facebook introduced a "Usernames" feature, whereby pages can be linked with simpler URLs such as https://www.facebook.com/facebook instead of https://www.facebook.com/profile.php?id=20531316728.^[146] Many new smartphones offer access to Facebook services through either their Web browsers or applications. An official Facebook application is available for the operating systems Android, iOS, and webOS. Nokia and Research In Motion both provide Facebook applications for their own mobile devices. More than 425 million active users access Facebook through mobile devices across 200 mobile operators in 60 countries.^[147]

The media often compares Facebook to Myspace, but one significant difference between the two Web sites is the level of customization.^[148] Another difference is Facebook's requirement that users give their true identity, a demand that MySpace does not make.^[149] MySpace allows users to decorate their profiles using HTML and Cascading Style Sheets (CSS), while Facebook allows only plain text.^[150] Facebook has a number of features with which users may interact. They include the Wall, a space on every user's profile page that allows friends to post messages for the user to see;^[151] Pokes, which allows users to send a virtual "poke" to each other (a notification then tells a user that they have been poked);^[152] Photos, where users can upload albums and photos;^[153] and Status, which allows users to inform their friends of their whereabouts and actions.^[154] Depending on privacy settings, anyone who can see a user's profile can also view that user's Wall. In July 2007, Facebook began allowing users to post attachments to the Wall, whereas the Wall was previously limited to textual content only.^[151]

On September 6, 2006, a News Feed was announced, which appears on every user's homepage and highlights information including profile changes, upcoming events, and birthdays of the user's friends.^[155] This enabled spammers and other users to manipulate these features by creating illegitimate events or posting fake birthdays to attract attention to their profile or cause.^[156] Initially, the News Feed caused dissatisfaction among Facebook users; some complained it was too cluttered and full of undesired information, others were concerned that it made it too easy for others to track individual activities (such as relationship status changes, events, and conversations with other users).^[157]

In response, Zuckerberg issued an apology for the site's failure to include appropriate customizable privacy features. Since then, users have been able to control what types of information are shared automatically with friends. Users are now able to prevent user-set categories of friends from seeing updates about certain types of activities, including profile changes, Wall posts, and newly added friends.^[158]On February 23, 2010, Facebook was granted a patent^[159] on certain aspects of its News Feed. The patent covers News Feeds in which links are provided so that one user can participate in the same activity of another user.^[160] The patent may encourage Facebook to pursue action against websites that violate its patent, which may potentially include websites such as Twitter.^[161]

One of the most popular applications on Facebook is the Photos application, where users can upload albums and photos.^[162] Facebook allows users to upload an unlimited number of photos, compared with other image hosting services such as Photobucket and Flickr, which apply limits to the number of photos that a user is allowed to upload. During the first years, Facebook users were limited to 60 photos per album. As of May 2009, this limit has been increased to 200 photos per album.^{[163][164][165][166]}Privacy settings can be set for individual albums, limiting the groups of users that can see an album. For example, the privacy of an album can be set so that only the user's friends can see the album, while the privacy of another album can be set so that all Facebook users can see it. Another feature of the Photos application is the ability to "tag", or label, users in a photo. For instance, if a photo contains a user's friend, then the user can tag the friend in the photo. This sends a notification to the friend that they have been tagged, and provides them a link to see the photo.^[167]

On June 7, 2012, Facebook launched its App Center to its users. It will help the users in finding games and other applications with ease.^[168] Since the launch of the App Center, Facebook has seen 150M monthly users with 2.4 times the installation of apps.^[169]The sorting and display of stories in a user's News Feed is governed by the algorithm EdgeRank.^[170]

Facebook Notes was introduced on August 22, 2006, a blogging feature that allowed tags and embeddable images. Users were later able to import blogs from Xanga, LiveJournal, Blogger, and other blogging services.^[42] During the week of April 7, 2008, Facebook released a Comet-based^[171] instant messaging application called "Chat" to several networks,^[172] which allows users to communicate with friends and is similar in functionality to desktop-based instant messengers. Facebook launched Gifts on February 8, 2007, which allows users to send virtual gifts to their friends that appear on the recipient's profile. Gifts cost $1.00 each to purchase, and a personalized message can be attached to each gift.^[173][174] On May 14, 2007, Facebook launched Marketplace, which lets users post free classified ads.^[175] Marketplace has been compared to Craigslist by CNET, which points out that the major difference between the two is that listings posted by a user on Marketplace are seen only by users in the same network as that user, whereas listings posted on Craigslist can be seen by anyone.^[176]

On July 20, 2008, Facebook introduced "Facebook Beta", a significant redesign of its user interface on selected networks. The Mini-Feed and Wall were consolidated, profiles were separated into tabbed sections, and an effort was made to create a "cleaner" look.^[177] After initially giving users a choice to switch, Facebook began migrating all users to the new version starting in September 2008.^[178] On December 11, 2008, it was announced that Facebook was testing a simpler signup process.^[179]

A new Messaging platform, codenamed "Project Titan", was launched on November 15, 2010. Described as a "Gmail killer" by some publications, the system allows users to directly communicate with each other via Facebook using several different methods (including a special email address, text messaging, or through the Facebook website or mobile app)—no matter what method is used to deliver a message, they are contained within single threads in a unified inbox. As with other Facebook features, users can adjust from whom they can receive messages from—including just friends, friends of friends, or from anyone.^[180][181]Aside from the Facebook website, Messages can also be accessed through the site's mobile apps, or a dedicated Facebook Messenger app.^[182]

Since April 2011, Facebook users have had the ability to make live voice calls via Facebook Chat, allowing users to chat with others from all over the world. This feature, which is provided free through T-Mobile's new Bobsled service, lets the user add voice to the current Facebook Chat as well as leave voice messages on Facebook.^[183]

On July 6, 2011, Facebook launched its video calling services using Skype as its technology partner. It allows one-to-one calling using a Skype Rest API.

On September 14, 2011, Facebook added the ability for users to provide a "Subscribe" button on their page, which allows users to subscribe to public postings by the user without needing to add them as a friend.^[184] In conjunction, Facebook also introduced a system in February 2012 to verify the identity of certain accounts. Unlike a similar system used by Twitter, verified accounts do not display a special verification badge, but are given a higher priority in a user's "Subscription Suggestions".^[185]In December 2012, Facebook announced that due to user confusion surrounding its function, the Subscribe button would be re-labeled as a "Follow" button—making it more similar to other social networks with similar functions.^[186]

To allay concerns about privacy, Facebook enables users to choose their own privacy settings and choose who can see specific parts of their profile.^[187] The website is free to users, and generates revenue from advertising, such as banner ads.^[188] Facebook requires a user's name and profile picture (if applicable) to be accessible by everyone. Users can control who sees other information they have shared, as well as who can find them in searches, through their privacy settings.^[189]According to comScore, an internet marketing research company, Facebook collects as much data from its visitors as Google and Microsoft, but considerably less than Yahoo!.^[190] In 2010, the security team began expanding its efforts to reduce the risks to users' privacy,^[191] but privacy concerns remain.^[192] On November 6, 2007, Facebook launched Facebook Beacon, which was an ultimately failed attempt advertise to friends of users using the knowledge of what purchases friends made. As of March 2012, Facebook's usage of its user data is under close scrutiny.^[193]

Since 2010 the National Security Agency has been taking Facebook profile information from users to discover who their allies, friends, and colleagues are.^[194]In August 2013 High-Tech Bridge published a study showing that links included in Facebook messaging service messages were being accessed by Facebook for its own purposes.^[195] In January 2014 two users filed a lawsuit against Facebook alleging that their privacy had been violated by this practice.^[196]

On November 29, 2011, Facebook agreed to settle US Federal Trade Commission charges that it deceived consumers by failing to keep privacy promises.^[197]

Facebook is built in PHP which is compiled with HipHop for PHP, a 'source code transformer' built by Facebook engineers that turns PHP into C++.^[198] The deployment of HipHop reportedly reduced average CPU consumption on Facebook servers by 50%.^[199]Facebook is developed as one monolithic application. According to an interview in 2012 with Chuck Rossi, a build engineer at Facebook, Facebook compiles into a 1.5 GB binary blob which is then distributed to the servers using a custom BitTorrent-based release system. Rossi stated that it takes approximately 15 minutes to build and 15 minutes to release to the servers. The build and release process is zero downtime and new changes to Facebook are rolled out daily.^[199]Facebook used a combination platform based on HBase to store data across distributed machines. Using a tailing architecture, new events are stored in log files, and the logs are tailed. The system rolls these events up and writes them into storage. The User Interface then pulls the data out and displays it to users. Facebook handles requests as AJAX behavior. These requests are written to a log file using Scribe (developed by Facebook).^[200]

Data is read from these log files using Ptail, an internally built tool to aggregate data from multiple Scribe stores. It tails the log files and pulls data out (thus the name). Ptail data is separated out into three streams so they can eventually be sent to their own clusters in different data centers (Plugin impression, News feed impressions, Actions (plugin + news feed)). Puma is used to manage periods of high data flow (Input/Output or IO). Data is processed in batches to lessen the amount of times needed to read and write under high demand periods (A hot article will generate a lot of impressions and news feed impressions which will cause huge data skews). Batches are taken every 1.5 seconds, limited by memory used when creating a hash table.^[200]

After this, data is output in PHP format (compiled with HipHop for PHP). The backend is written in Java and Thrift is used as the messaging format so PHP programs can query Java services. Caching solutions are used to make the web pages display more quickly. The more and longer data is cached the less realtime it is. The data is then sent to MapReduce servers so it can be queried via Hive. This also serves as a backup plan as the data can be recovered from Hive. Raw logs are removed after a period of time.^[200]

The like button is a social networking feature, allowing users to express their appreciation of content such as status updates, comments, photos, and advertisements. It is also a social plug-in of the Facebook Platform – launched on April 21, 2010^[201][202] – that enables participating Internet websites to display a similar like button. Following the termination by the sheriff of Hampton, Virginia, US of employees who liked the Facebook page of an adversary, a federal appeals court in Virginia handed down a decision that the US Constitution protects the rights of US citizens to like any Facebook page of their choosing. US Circuit Judge William Traxler likened the practice to displaying a "political sign in one’s front yard."^[203]

Patents relating to the "Like" button and other social features held by deceased Dutch programmer Joannes Jozef Everardus van Der Meer are subject of a lawsuit brought against Facebook by Rembrandt Social Media.^[78][204] Rembrandt is represented by the Fish & Richardson Law Firm that stated "We believe Rembrandt's patents represent an important foundation of social media as we know it, and we expect a judge and jury to reach the same conclusion based on the evidence." As of April 2, 2013, further information about the case is unknown in the public sphere.^[205]

According to comScore, Facebook is the leading social networking site based on monthly unique visitors, having overtaken main competitor MySpace in April 2008.^[206] ComScore reports that Facebook attracted 130 million unique visitors in May 2010, an increase of 8.6 million people.^[207] According to Alexa, the website's ranking among all websites increased from 60th to 7th in worldwide traffic, from September 2006 to September 2007, and is currently 2nd.^[208] Quantcast ranks the website 2nd in the U.S. in traffic,^[209] and Compete.com ranks it 2nd in the U.S.^[210] The website is the most popular for uploading photos, with 50 billion uploaded cumulatively.^[211] In 2010, Sophos's "Security Threat Report 2010" polled over 500 firms, 60% of which responded that they believed that Facebook was the social network that posed the biggest threat to security, well ahead of MySpace, Twitter, and LinkedIn.^[191]

Facebook is the most popular social networking site in several English-speaking countries, including Canada,^[212] the United Kingdom,^[213] and the United States.^{[214][215][216][217]} However, Facebook still receives limited adoption in countries such as Japan, where domestically created social networks are still largely preferred.^[218] In regional Internet markets, Facebook penetration is highest in North America (69 percent), followed by Middle East-Africa (67 percent), Latin America (58 percent), Europe (57 percent), and Asia-Pacific (17 percent).^[219] Some of the top competitors were listed in 2007 by Mashable.^[220]

The website has won awards such as placement into the "Top 100 Classic Websites" by PC Magazine in 2007,^[221] and winning the "People's Voice Award" from the Webby Awards in 2008.^[222] In a 2006 study conducted by Student Monitor, a New Jersey-based company specializing in research concerning the college student market, Facebook was named the second most popular thing among undergraduates, tied with beer and only ranked lower than the iPod.^[223]On March 2010, Judge Richard Seeborg issued an order approving the class settlement in Lane v. Facebook, Inc.,^[224] the class action lawsuit arising out of Facebook's Beacon program.

In 2010, Facebook won the Crunchie "Best Overall Startup Or Product" for the third year in a row^[225] and was recognized as one of the "Hottest Silicon Valley Companies" by Lead411.^[226] However, in a July 2010 survey performed by the American Customer Satisfaction Index, Facebook received a score of 64 out of 100, placing it in the bottom 5% of all private-sector companies in terms of customer satisfaction, alongside industries such as the IRS e-file system, airlines, and cable companies. The reasons why Facebook scored so poorly include privacy problems, frequent changes to the website's interface, the results returned by the News Feed, and spam.^[227]